At the present time a number of publications are available describing increasing the accuracy of measuring systems by improving the hardware. The development of digital methods and hardware for radio measurements permit a very close approach to the solution of the problem of increasing the accuracy of measuring systems.
Phase heterodyne systems with incoherent radiation of signals are characterized by low stability of the frequencies produced by ground stations. These systems include "Poisk" (USSR), "Torane" (France), "Lorac" (USA). Due to the low frequency stability of the radiation, it is difficult to use a digital measuring structure, since the meter must include sophosticated systems to form a copy of the signal. A simpler technical solution is to provide electromechanical follow-up systems making it possible to measure the difference of the phases of the signals. The follow-up systems are included in the radio system. The compensation phase meters based on an electromechanical system are widely used in the art. Such meters can operate with radio systems having low frequency stability. However, the accuracy and reliability of such measuring devices is low, since it is determined by the characteristics of such mechanical units as reduction gears, gear wheels and toothed and worm transmissions.
The input of information directly from the meter to the computer processing the navigation information is very difficult. The presence of additional mechanical transducers converting the mechanical action into a digital code still further deteriorates the accuracy of measurement of a navigation parameter.
Known in the art is a mobile receiver station of a phase radio navigation system "Lorac" of the Seiscor Company consisting of receivers and an indicator unit based on a phase meter of the compensation follower type with a phase shifter using a selsyn, a motor, a phase detector and a mechanical transmission. The radio navigation parameter is fed to the computer via an analog-code converter. This receiver station permits the measurement of the phase difference in the radio system having low frequency stability of the ground stations. However, the accuracy of the measurements is low due to the use of mechanical components.
Known in the art is a receiver-indicator of the Sercel Company (France). This receiver-indicator includes an input unit connected to an antenna and an output connected to the inputs of RF amplifiers whose output is connected to IF amplifiers through mixers. The other inputs of the mixers are connected to a master oscillator and the outputs of the IF amplifiers are connected through a switch to the first inputs of the readout follower systems whose second inputs are connected to the output of a timing unit. The input of the timing unit is connected to the master oscillator. The outputs of the readout follower systems are electrically connected to the inputs of the phase meters through a switching device.
The aforedescribed receiver-indicator permits accurate measurements, since it is based on a digital measuring structure. However, the measurements can be made only at strictly definite frequencies with high stability of radiation of the ground stations included in the radio system. The use of such a measuring structure for measuring the difference of phases from the signals of ground stations having low frequency stability of radiation would cause great measuring errors.
Well known in the art is a two-channel receiver-indicator for phase radio systems (cf. V. P. Mayko "Requirements Imposed on R-F Phase Meters," "Geodezia i Aerofotos'emka" No. %, 1977, published by Izvestia Vashikh Uchebnykh Zavedeni Publishing House), comprising three receivers, two of which are tuned respectively to the frequencies of the first guided and guiding stations and the second guided and guiding stations of the radio system and are connected to the input of the first signal shaper for shaping electric signals corresponding to the beat frequency between the first guided and guiding stations and to the input of the second signal shaper for shaping electric signals corresponding to the beat frequency between the second guided and guiding stations respectively. The third receiver is tuned to the frequency of the repeater station of the radio system and is connected to the input of the third signal shaper for shaping electric signals corresponding to the beat frequency between the first guided and guiding stations and to the input of the fourth signal shaper for shaping electric signals corresponding to the beat frequency between the second guided and guiding stations. The outputs of the first and third signal shapers are connected to the inputs of a first follow-up phase meter and the outputs of the second and fourth signal shapers are connected to the inputs of the second follow-up phase meter.
In this receiver-indicator, the first and second phase meters measuring the phase difference are electromechanical followers of the compensation type. A phase shifter is a measuring element in the follow-up system. The electromechanical follower comprises a phase detector electrically connected to a motor and a phase shifter, which are mechanically interconnected. The error signal from the output of the phase detector controls the operation of the motor, which rotates the rotor of the phase shifter until the error signal disappears. The angle of rotation of the phase shifter indicates the phase difference.
The aforedescribed receiver-indicator is low in reliability and accuracy due to application of such complex mechanical units as reduction gears, motors and phase shifters included in the electromechanical system.